Apparatus for ultrasonic cleaning



June 6, 1961 N. G. BRANSON I 2,987,068

APPARATUS FOR ULTRASONIC CLEANING Filed May 1, 1956 2 Sheets-Sheet 1 F F/GZ /2 I M Q s ,o O

INVENTOR.

A/armon 6. 5/0/7500 BY W, Z/ z Yaw A f/om eys June 6, 1961 N. G. BRANSON 2,987,068

APPARATUS FOR ULTRASONIC CLEANING Filed May 1. 1956 2 Sheets-Sheet 2 FIG. 4

INVENTOR. Norman 6. Branson tats This invention relates to apparatus for cleaning in a liquid medium wherein ultrasonic waves are generated.

A type of cleaning procedure has been developed wherein high frequency mechanical vibratory energy is produced in a liquid bath. The improvement in cleaning over that produced in a bath of the liquid alone results from the mechanical scrubbing action which is produced when the liquid is vibrated at a high frequency. The vibrations are produced in the liquid by a suitable transducer operating according to well known piezoelectric principles and powered from a suitable generator. The transducer may be submerged in the liquid or have a surface in contact with it, as for example a wall of a container or tank. The range of frequencies which may be used extends all the way from 4 to over 400 kilocycles but a frequency in the range of about 36 to 40 kilocycles is generally preferred. Ultrasonic cleaning is particularly useful in industry in the cleaning of machined metal parts, which are usually covered with oil and accumulations of dirt, chips, lapping compound, bits of abrasive and the like, which must be removed before the parts are assembled or put to use. Ultrasonic vibrations, added to conventional metal cleaning procedures, result in faster and more thorough cleaning, a saving in labor costs and reduction of rejects and improvement in quality generally.

It has been observed that in order to obtain good cleaning action the intensity of vibration of the radiating surface of the transducer must be high enough to produce cavitation in the cleaning solution. It is the cavitating liquid which provides an erosive action which is the equivalent of scrubbing. During the tension phase of the ultrasonic wave, bubbles are formed within the liquid wherever there are imperfections, microscopic nuclei, or adsorbed air. Some grow large enough to pass off from the liquid resulting in degassing. then contract quietly in the compression phase. If the size after expansion exceeds a critical ratio to the initial size, the bubble will be burst and crash or cavitate, and it is only these that contribute a cleaning effect. Energy used for expansion of the non-cavitating bubbles is wasted. The energy input that will maintain cavitation, once the cavitation threshold has been reached, is about half the initial cavitation values. If the intensity of the current which drives the transducer is too high, however, excessive cavitation occurs at the face of the transducer. This is undesirable for two reasons. The layer of bubbles produced on the face of the transducer reduces the energy available in the liquid because it distorts the plane parallel wave front and the energy dissip-ates or scatters. Furthermore, the cavitation on the face of the transducer erodes it and wears it away in a relatively short time. It is therefore better to hold down the energy intensity imposed on the transducer to a level at which there is no cavitation or only a moderate cavitation at its face and an undistorted wave front. This being a limitation, one method which has been used to increase the scrubbing action on the parts to be cleaned is to so shape the transducer or so arrange a plurality of transducers as to focus the ultrasonic waves on the cleaning area wherein intense cavitation can be produced with little or no cavitation at the transducer face. This, however, merely accentuates a difficulty encountered in this Some expand and Patented June 6, 1961 type of cleaning known as shadowing. The energy beams travel from the transducers in relatively narrow paths and they have a scrubbing action only on that side of the article to be cleaned which is directed toward the transducer, yet every portion of a piece to be cleaned must be exposed to the scrubbing or erosive action within the cavitation zone for a time long enough to clean it completely. This has heretofore required that the parts being cleaned must be moved around or rotated while they are in the bath so as to be subjected to scrubbing action on all sides.

The general object of the present invention is to overcome this difliculty and to provide an apparatus wherein articles are subjected to a scrubbing action by the ultrasonic waves on all sides in a bath vibrated by ultrasonic transducers which are positioned only on one side of the articles immersed in or passing therethrough. This object is accomplished by utilizing the discovery that the ultrasonic waves are reflected from a liquid surface and that these reflected waves may be utilized in obtaining the desired scrubbing action. I have found it to be most effective to use a submerged transducer arranged to produce a focus which is substantially outside of the liquid surface of the bath, thereby producing an inverted focus of reflected waves between which and the liquid surface there is a zone readily adapted for cleaning in which there is a most intense cavitation produced on the one hand by waves coming directly from the transducer elements and, on the other hand, by Waves reflected from the liquid surface, these two sets of waves approaching an article within this zone from opposite sides and diverse directions.

In the drawings:

FIG. 1 is a diagrammatic representation of my presently preferred inverted focus arrangement;

FIG. 2 is another arrangement using a pair of unfocused banks of transducers and reflected waves;

FIG. '3 shows a further arrangement utilizing a single unfocused bank of transducers in a similar manner; and

FIG. 4 illustrates an alternative arrangement in which focused ultrasonic energy waves may be reflected from a wall-supported liquid surface in accordance with a specific embodiment of the invention.

In FIG 1 the tank, which may be of any desired shape and size, is shown at '10. It is filled with a suitable liquid 11, water being one of the best, to such a level as to form the liquid surface 12. In the bottom of the tank and mounted on any suitable support 14 having a concave upper surface, there is a plurality of transducer elements 15. Such elements are well known in the art and may, for example, be hermetically sealed stainless steel metal cans of generally rectangular shape within which there are piezoelectric elements such as those formed of barium titanates, these elements being adhered to the upper surfaces of the cans so that their vibrations, when subjected to high frequency energy,are

transmitted into the liquid. Such elements may, for example, be about 2 inches in width, 3 inches high, and presently available elements vary in length from 8 /2 inches to 14 /2 inches, providing radiating surfaces of 2 /2 x 6 inches or 2 /2 x 12 inches. The size and type used will depend upon the size of the tank in which the cleaning is taking place, and it is to be understood that size is arbitrary and not a limitation of the invention. In FIG. 1 the transducer elements are seen in end view. The curvature of support v14 is such as to produce a focus F of the waves produced in the liquid which is well beyond the liquid surface 12. The direct paths taken by these waves are shown by the straight lines 16, these wave paths being for the most part reflected back from the liquid surface as shown at 16 to a common inverted focus F. The area of maximum scrubbing action and cavitation is delineated by the rectangle 18 in broken lines which may be referred to as the cleaning zone. It will be seen that any article within this zone will 'be attacked by waves emitted directly from the transducer elements 15 from below and also from above at various angles by the waves reflected from the liquid surface '12. Since the liquid surface is almost never entirely flat, there will also be a certain amount of scattering action on the reflected waves and they will attack at even more diverse angles than those shown in the drawing.

It will be understood that articles to be cleaned will be suspended or passed through the cleaning zone in any suitable manner. According to one usual practice, they will be carried in openwork baskets on a conveyor which drops the baskets into the bath at one end, carries it horizontally through the bath and through the cleaning zone to the other side of the bath from which it is then removed.

While transducer elements have been focused heretofore in a manner similar to that illustrated in FIG. 1 by arranging them on an arcuate support, it has been heretofore the practice to so relate the transducers to the liquid level that the focus F is well below it. In such an arrangement, the articles to be cleaned have been passed through the area surrounding the focus F and shadowing has been particularly pronounced, very little, if any, of the mechanical energy produced by the 2,9e7,0es

the arrow, will be subjected to a scrubbing or erosive action on all sides.

FIG. 3 shows a similar arrangement using only a single bank of transducer elements 15 in a tank of such ultrasonic waves reaching the articles by reflection from the liquid surface due to the fact that any reflected waves travel so far as to be ineifective in cleaning. It will be observed from FIG. 1 that according to the invention the liquid level 12 is so related to the focus F of the transducer elements that the inverted focus F is a substantial distance below the surface. The particular location of the inverted focus F may be adjusted throughout a considerable range without departing from the spirit of the invention. The lower it is placed, the greater will be the size of the cleaning zone 18 within which scrubbing action takes place on articles from opposite sides.

A modification of the arrangement shown in FIG. 1 which may be used is to position the transducer 15 at one side of the tank directed toward an opposite wall thereof in such a position that the focal point F lies substantially beyond the wall so that the energy waves are reflected back toward an inverted focus F well within the body of liquid 11 in'the tank. It will be understood that the surface of the liquidfrom which the energy waves are reflected is not necessarily a free surface since the same phenomenon occurs when the surface of the liquid body is one in contact with a flat container wall.

FIG. 2 shows a modification in which the transducer units '15 are arranged in a flat bank, two such banks being used, mounted on supports 14a and so directed toward the liquid surface 12that the energy waves are directed angularly therefrom back into the body of liquid. Here again articles passed through the bath, as for example in a basket 20, passing in the direction shown by size that the basket 20 may be passed from left to right so as to be subjected to energy waves first from below and then from both above and below and then from above only and finally pass into a quiescent or rinsing zone from which it may be removed.

It is to be understood that the invention as hereinafter defined in the claims is not limited to the details above described merely for purposes of illustration.

What is claimed is: I

1. Apparatus for ultrasonic cleaning comprising: a tank, a body of liquid therein having a surface, transducer means adapted to generate waves of mechanical vibratory energy in said body at ultrasonic frequency, said transducer means being focused to direct the waves of energy produced in the liquid toward a focal point substantially beyond said surface, whereby said waves are reflected back toward an inverted focal point substantially within said body of liquid, and means for supporting an article to be cleaned in the zone traversed by both the direct and the reflected energy waves.

2. Apparatus according to claim 1 wherein said transducer means comprises a plurality of units each adapted to produce a beam of ultrasonic energy, said units being arranged on an arc. 7

3. Apparatus according to claim 1 wherein said liquid surface is the free horizontal surface of the liquid in the tank.

4. Apparatus according to claim 1 wherein said liquid surface is a surface supported by a wall of said tank.

5. Apparatus according to claim 4 wherein the said wall of the tank is flat so as to support the liquid surface reflecting said waves in a plane.

6. Apparatus for ultrasonic cleaning comprising, in combination, a tank, a body of liquid therein having a free horizontal surface, two transducer means positioned below said surface within said body and adapted; to generate beams of ultrasonic energy directed angularly toward said surface to be reflected therefrom back into the liquid body, and means for supporting an article in said liquid so as to be subjected to both the direct and reflected beams, said transducer means being so posi tioned and spaced apart that their direct beams conv verge toward said surface and their reflected beams intersect within the body of liquid.

' References Cited in the file of this patent UNITED STATES PATENTS w ne-i w, 

